1. Field of the Invention
The present invention relates to steel with improved impact penetration resistance and to a method for producing the steel. In particular, the present invention relates to a steel with improved impact penetration resistance, weldability and bendability.
2. Discussion of the Background
Steel plate is required in certain applications to exhibit impact penetration resistance to projectiles such as shell splinters, bullets or space debris. To achieve impact penetration resistance, it is common to modify the chemical composition of the steel plate and to perform a heat treatment such as aging on the steel plate to increase its strength.
To produce a member having impact penetration resistance, it is conventional to cast an ingot and to form the ingot into a slab. The slab is then hot rolled to a required thickness and cooled. Then the slab is cut into pieces small enough for batch heat treatment, is processed for bending as required, and is subjected to a batch heat treatment such as aging to form a steel plate product having impact penetration resistance.
When large members are to be produced, smaller members are welded together after having been bent as required and subjected to batch heat treatment such as aging.
The tensile strength of conventional high-tensile or maraging steel plates designed for impact penetration resistance is suppressed to no more than 1200 MPa to avoid defects leading to delayed fracture of the plates in practical use.
However, these conventional steel plates have a problem in that the impact penetration resistance of the plates is inadequate. In particular, the steel plates have a penetration border energy ratio (as defined below) of only 1.5 relative to JIS SS400 (JISG3101 (1995)) plain carbon steel serving as a reference steel. JIS SS400 (JISG3101 (1995)) steel has a tensile strength of 400-510 MPa (N/mm2) and corresponds to ASTM A36.
An additional problem with these conventional steel plates is that, since the plates require batch heat treatment, their allowable maximum size must be smaller than the dimensions of heat treatment furnaces.
Another problem with these conventional steel plates is that when steel plates that have been subjected to heat treatments such as ageing are welded together, preheating is required to prevent the formation of cold cracks. If the steel plates are welded together without preheating, then expensive austenitic welding materials are needed, resulting in an increase of cost.
Yet another problem with these conventional steel plates is that after having been subjected to heat treatment such as aging, the steel plates have poor bending properties and require a large bend radius (4 t) equal to four times the thickness t. To avoid this problem, the plates must be bent before being subjected to a heat treatment such as aging, which restricts flexibility in processing.
In view of the above, an object of the present invention is to improve the impact penetration resistance, and weldability and bendability, of steel.
In order to solve the aforementioned problems, the present inventors have conducted impact penetration tests with projectiles on a wide variety of steel from soft steel to high-strength steel, using a compact impact test apparatus for evaluating impact penetration resistance. However, the tests revealed the impossibility of improving impact penetration resistance only by increasing tensile strength.
The inventors then researched the influence of deformation, hardness distribution and microstructure of materials on impact penetration resistance, and introduced a new concept of penetration border energy. As a result of penetration tests measuring the penetration border energy of various materials, the inventors found that the penetration border energy of steel is enhanced if the tensile strength of the steel is in a certain range above a certain value and the yield ratio is lowered. Thus, the invention was completed successfully.
The term xe2x80x9cyield ratioxe2x80x9d as used herein refers to the ratio of yield strength to tensile strength (i.e. yield strength/tensile strength).
The term xe2x80x9cpenetration border energyxe2x80x9d as used herein refers to the kinetic energy of a projectile traveling at the maximum speed at which the projectile will fail to penetrate a steel sample.
The invention was made to solve the aforementioned problems associated in the prior art.
A first aspect of the invention provides a steel with improved impact penetration resistance that has a tensile strength of 850 to 1700 MPa, a yield ratio of at most 80%, and a penetration border energy ratio of at least 2.0 relative to a JIS SS400 reference steel of the same thickness. This steel can have improved impact penetration resistance, weldability and bendability, because aging and similar heat treatments are not needed.
The term xe2x80x9cpenetration border energy ratioxe2x80x9d as used herein refers to the ratio of the penetration border energy of a steel sample to the penetration border energy of a JIS SS400 reference steel sample, where the two penetration border energies are determined in penetration tests conducted under identical conditions. In particular, the penetration tests are conducted with a steel sample and a JIS SS400 reference steel sample having the same thickness. The penetration border energy ratio is used instead of the penetration border energy to draw general conclusions regarding impact penetration resistance, because absolute values of penetration border energy depend upon penetration test variables such as the shape and hardness of the projectile.
A second aspect of the invention provides a steel with improved impact penetration resistance that, in addition to the features of the steel of the first aspect of the invention, has at most 0.15% by weight of C and a weld crack sensitivity Pcm of at most 0.27, where Pcm=C+Mn/20+Si/30+Ni/60+Cr/20+Mo/15+V/10+Cu/20+5B (% by weight, respectively).
A third aspect of the invention provides a method for producing steel with improved impact penetration resistance from unstable austenitic steel. The term xe2x80x9cunstable austenitic steelxe2x80x9d as used herein refers to a steel in which austenite is not the predominant phase at room temperature. The method according to the third aspect of the invention includes the steps of first effecting a heat treatment 1 on a steel; then effecting, at least once, one or more or any combination of the heat treatment 1 and a heat treatment 2 on the steel; and then finally effecting the heat treatment 2 on the steel. The heat treatment 1 includes heating the unstable austenitic steel to at least the Ac3 transformation temperature and then water-cooling the steel to a temperature below 350xc2x0 C. The heat treatment 2 includes heating the steel to a temperature between the Ac3 and Ac1 transformation temperatures and than water-cooling the steel to a temperature below 350xc2x0 C. The Ac1 transformation temperature is the temperature at which austenite begins to form during heating, and the Ac3 transformation temperature is the temperature at which transformation of ferrite to austenite is completed during heating.
According to the third aspect of the invention, steel with improved impact penetration resistance can be produced, even from unstable austenitic steel, that has a tensile strength of 850 to 1700 MPa, a yield ratio of at most 80%, and a penetration border energy ratio of at least 2.0. The microstructure of the steel after the final heat treatment 2 includes bainite, martensite and island-shaped martensite in combination.
Embodiments of steel with improved impact penetration resistance according to the invention will now be described in greater detail.